AbstractA study of the ultrastructural characteristics of an intracellular bacterium infecting the redclaw crayfish, Cherax quadricarinatus, a pathogen referred to previously as a rickettsia-like organism (RLO), revealed the presence of different developmental stages. These included a rod-shaped and uniformly electron-dense elementary body (EB) and an intermediate body (IB). The length of the EB varied between 0.48 and 0.6 microm, and the diameter was 0.3 microm. The IB was 0.75 to 1.1 microm long by 0.36 to 0.44 microm in diameter. Although the EB of this bacterium has ultrastructural characteristics similar to those of Rickettsiella, no information is available regarding its genetic relationship to this genus, and the intracellular bacterium should continue to be referred to as a rickettsia-like organism. The hemocytes had different levels of infection, and the RLO proliferated inside these cells. The EB appeared to be free in the cytoplasm of infected hemocytes and other cells; however, this might be a fixation artifact. The EB was also contained in membrane-bound vacuoles along with the IB. RLO colonies were observed inside small granular cells. No large granular cells were observed in the sections examined; therefore, no data were obtained regarding infection of this type of hemocyte. The fixed phagocytes on the external side of the terminal hepatic arterioles had an activated interrupted layer containing RLO bacteria. Stem cells in the hematopoietic tissue were also infected, and some cells were apparently being released into circulation.